Murine monoclonal antibodies have been generated which demonstrate reactivity with human mammary tumor cells and not with apparently normal tissues tested. The monoclonals could be placed into five major groups based on their differential binding to the surface of live mammary tumor cells in culture, extracts, or to tissue sections of mammary tumors employing the immunoperoxidase technique. Lymphocytes from lymph nodes obtained at mastectomy in breast cancer patients have been fused with murine non-Ig producer myeloma cells to obtain human-mouse hybridoma cultures that synthesize human monoclonal antibodies. One human monoclonal IgM was used in the immunoperoxidase technique to discriminate between normal and mammary carcinoma cells from 55 of 59 patients. Monoclonal antibodies have been generated against both the major internal protein and external glycoprotein of the mouse mammary tumor virus. These antibodies have been used in the murine model to define the heterogeneity of expression of given gene products that may occur among mammary tumors of a given species and, at times, in different areas of the same mammary tumor.